In a typical magnetic resonance system for imaging or spectroscopy, one radio frequency power amplifier is used for the transmit phase (that is, for magnetic resonance excitation). The output of the amplifier is fed into two channels of a quadrature “whole body” transmit coil, namely into the 0° phase “I” channel and the 90° phase “Q” channel. Coupling of the amplifier with the I and Q channels of the quadrature transmit coil is typically accomplished using a so-called “hybrid” coupler, which introduces a 90° phase shift for the Q channel, and uses a load for reflected power.
Another type of coil is a multi-element body coil. Such a coil includes a plurality of independently drivable conductors that can be driven in various ways by a corresponding plurality of radio frequency power amplifiers to provide substantial control over the transmit B1 field, so as to accommodate different subject loads and other factors. Such a multi-element body coil can be constructed, for example, as a degenerate birdcage coil, or as a set of rods connected with a radio frequency screen so as to be drivable in a transverse electromagnetic (TEM) mode. More generally, one can employ a multi-channel radio frequency coil, such as a multi-element body coil or an array of surface coils or other local coils, to generate a highly spatially tunable B1 transmit field.
Multi-element body coils coupled with a corresponding multiple number of radio frequency power amplifiers represent a substantial increase in system complexity and cost as compared with a quadrature body coil driven by a single power amplifier via a hybrid coupler. Accordingly, in some applications it is desired to drive a multi-channel radio frequency coil using a single radio frequency power amplifier. For example, a multi-element body coil can be driven in a quadrature operating mode using a single radio frequency power amplifier and suitable power coupling circuitry.
However, heretofore it has been found that suitable power coupling circuitry is complex. One suitable power coupler is known as a Butler matrix. For driving an N-channel multi-element body coil in quadrature operating mode, a Butler matrix circuit includes at least N/2+N/4+ . . . +N/N hybrid couplers combined with loads and cables of defined length. For example, a Butler coupling matrix configured to drive an 8-channel multi-element body coil in quadrature requires 8/2+8/4+8/8=7 couplers in the Butler matrix. The Butler matrix also exhibits substantial power loss, and is complex to construct because each of the N/2+N/4+ . . . +N/N couplers and the corresponding cable lengths have to be adjusted to achieve the requisite impedance and phase matching.
The following provides new and improved apparatuses and methods which overcome the above-referenced problems and others.